The goals of the proposed research are (1) to define the functions of the Punch locus of Drosophila melanogaster in the developing nervous system and (2) to use this genetically well-characterized locus to initiate a dissection of regulatory components necessary for these functions by searching for genes and gene products that interact with Pu. The first goal will be achieved through a characterization of Pu transcript and protein expression in neural cells, elucidation of the structure of the neural products of Pu and the examination of the morphological and biochemical effects of mutations in Pu on neural development. The second goal will be achieved by searching for possible interactions between Pu and known genes that have patterns of expression or mutant phenotypes that parallel those of Pu and by performing genetic screens for suppressors and extragenic concomplementors of particular Pu mutations. Punch encodes the pteridine biosynthetic enzyme, GTP cyclohyrolase, which catalyzes the first step in the expression of the regulatory cofactor, tetrahydrobiopterin. Tetrahydrobiopterin. Tetrahydrobiopterin is required for an modulates the expression of the catecholamines and serotonin. In humans, mutations causing loss of GTP cyclohydrolase activity are characterized by failure to synthesize these neurotransmitters, resulting in early death. Work in mammals also implicates this cofactor in regulatory interactions in cell signaling systems operating in immune and neural cells. These roles appear to be the basis for very high levels of pteridines in the serum, urine and cerebrospinal fluid of patients suffering from HIV-associated neurological symptoms and a variety of proliferative diseases. Our work reveals strongly homologous functions in Drosophila and mammals and a high degree of conservation between the sequence of mammalian and Drosophila GTP cyclohydrolase. It is anticipated that the identification of other components in Drosophila that participate in functional and regulatory interactions involved in neural processes requiring pteridines will lead to a better general understanding of this important feature of neural development.